SLAC Accelerator Development Program: X -Band RF Power Sources

1. SLAC Accelerator Development Program: X-Band RF Power Sources Michael V. Fazio OHEP Accelerator Development Review January 24-26, 2011

2. Outline Goals and relevance to SLAC/DOE missions Historical perspective on SLAC high power X-band source development Issues limiting X-band source performance Future plans Collaborations and commercialization Summary SLAC Accelerator Development Program Page 2

3. Project Goals Support SLAC and DOE Mission Serving the National Interest • Overall Project Goals • Understand the physical mechanisms limiting the performance of high power X-band (11-12 GHz) klystrons • Use the knowledge to improve power, pulse width, and reliability as accelerator power sources. • Increase availability of commercial X-band technology for use in advanced accelerator applications • Relevance to SLAC/DOE Mission • Nurtures the 45+ years of SLAC’s unique high power RF experience in support of advanced accelerator technology for DOEs HEP research program • Advances high power RF source S&T in support of new mission applications • Compact accelerators • Advanced x-ray and gamma-ray photon sources • Specialized components (linearizers, RF deflectors) SLAC Accelerator Development Program Page 3

4. SLAC Has Two Decades of Experience with High Power X-Band Technology 1990’s through 2004 R&D effort to develop an X-band klystron for the proposed Next Linear Collider Culminated in the development of the solenoid focused XL series of tubes capable of 50 MW in a 1.5 µs pulse and 90 MW at 100 ns pulse width. A periodic permanent magnet focused klystron, the XP series, was also designed and tested, and demonstrated 75 MW at 1.6 µs at 120 Hz. SLAC Accelerator Development Program Page 4

5. More than 40 klystrons have been built and tested including 18 XL4s and 5 XL5s SLAC Accelerator Development Program Page 5

6. XL-4 Klystron Has Been the Workhorse with Some Operating a Few 10,000s of Hours Mostly Below 35 MW • X-band RF source tube for NLCTA, ASTA , LCLS, and RF breakdown experiments. • XL4/5 is last iteration of a line of 100 and 50 MW solenoid focused tubes. • To date 18 tubes have been produced with several tubes having over 10k hrs of filament time. • Performance specifications • 11.424 GHz • 50 MW peak, 9 kW average • 1.5µs @ 60Hz • Solenoid focusing We do not know long term reliability at 50 MW SLAC Accelerator Development Program Page 6

7. 5 XL5s Built for PSI, SincrotroneTrieste, & CERN • XL4 variation for higher frequency 11.992 – 11.994 GHz • Same gun, magnet, drift tube, collector and in general same cavity and waveguide mechanical layout • Performance Specifications • 50 MW, 1.5us, 100 Hz • A few changes from XL4 to reduce operating gradients and increase reliability • Cavities re-tuned, output structure improved • Improved output coupler, bends etc. • New window design • Rounded edges • After processing and operation at ≥50MW at 60Hz for 7 hours, detected no rf breakdown in the tube during a 24 hour heat run. • XL5 work is directly funded by customers as “Work for Others” and is still ongoing SLAC Accelerator Development Program Page 7

8. Observe Breakdown and RF Pulse Shortening as Power and Pulse Length Increase In XL4 9 events occurred during 17 hrs running at 50 MW with 1.44 us pulse width. • Breakdown events damage the output structure • Lifetime limitation issues are not understood • At 75 MW, iris surface field ~ 70 MV/m • Lower than in 3% vg accelerating structures • but higher than sustainable (~ 50 MV/m) in waveguide with comparable vg (~ 20%) as the klystron TW output structures. • Need to understand the difference between RF-induced breakdown and electron-beam induced breakdown SLAC Accelerator Development Program Page 8

9. Tube Autopsies Show Output Waveguide Coupler Iris Damage from RF Pulse Heating Waveguide coupling iris XL4-12 50 MW XL-PPM 75 MW XP1 SLAC Accelerator Development Program Page 9

10. SEM Photos of a 75 MW PPM X-band Klystron Output Section Showing Surface Damage Page 10

11. Developed Approach to Test New Output Structures without Building a Whole Klystron Approach uses an RF Driven Output Structure Reduces cost and turnaround time for testing new designs Should prove useful for comparing different structures Caveat: The power enters the cavity differently than when coupled from a bunched electron beam. Power flow and field pattern will not be the same. SLAC Accelerator Development Program Page 11

12. Pulse Heating Caused by Magnetic Field Driven Surface Currents Are Also an Issue Test Structures Thin Lip 0.024inch (0.6096mm) Thick Lip 0.08inch (2.032mm) SLAC Accelerator Development Program Page 12 • Pulse Heating Estimation for 75 MW, 400 ns Pulses: • ΔT ~ 112 ºC for thin lip, Bmax=0.82Tesla • ΔT ~ 31ºCfor thick lip, Bmax=0.43Tesla • Emax = 86 MV/m for thin lip • Emax = 83 MV/m for thick lip

13. Larger Iris Radius Improves Breakdown Rate by ~ Order of Magnitude XL4 Klystron at ~ 1000 ns SLAC Accelerator Development Program Page 13

14. We Observe that High Temp. H2 Brazing (>1000 C) and Bakeout Improves Surface and RF Performance • Heat treatment is a key to improving performance but not quantified • Need to investigate Surface Topography of Copper Cavity Noses (Magnification: 1000x) 750 C Heat Treatment Before Etch After Chemical Etch After Heat Cycle SLAC Accelerator Development Program Page 14 L. Laurent, Advanced Accelerator Concepts, July 2006

15. Future Plans MAGIC“simulation of 75MW-XP3 illustrates typical beam behavior Lifetime testing the (thick-lipped) XL4 Develop new ideas for more robust output structures Continue testing rf-powered output sections to see if they have similar breakdown characteristics as beam-powered sections In FY12 planning to build an XL4 with a demountable output structure for rapid test turnaround to enable a variety of structures to be tested under realistic beam conditions Enables us to confirm results from the RF-driven output section tests under realistic beam conditions SLAC Accelerator Development Program Page 15

16. Collaborating with numerous institutions to further the development of X-band high power sources CERN, Sincotrone Trieste, PSI, Brookhaven, and Lawrence Livermore Livermore - compact 250 MeV Compton gamma-ray source funded by the Dept. of Homeland Security. MEGa-ray, uses a SLAC built X-band linac system with an RF photoinjector. Los Alamos National Laboratory - pre-conceptual design of a 50 keV x-ray FEL light source for the Matter and Radiation Interaction in Extremes (MaRIE) Project, based on X-band RF and linac technology developed at SLAC. CERN - SLAC is developing a staged approach to a multi-TeV linear collider where the first stage relies on an X-band klystron based linac. Could reduce the risk associated with the Two Beam Accelerator technology that CERN is developing resulting in an earlier turn-on of a linear collider. LCLS utilizes an X-band structure powered by an XL klystron as a longitudinal phase space linearizer as part of its routine operation. Similar Linearizers using XL klystrons are being constructed for XFEL projects at Trieste, FERMI, and PSI and for the Brookhaven ATF. SLAC Accelerator Development Program Page 16

17. Commercialization Effort Underway Commercial availability of the klystron is perceived as the major limitation to the use of X-band technology Request for Proposals was advertised in 2010 for commercial procurement of an XL type klystron. Three Responses were received and are in the evaluation process. Will procure 1 or 2 depending on cost. SLAC Accelerator Development Program Page 17

18. Summary Demand for high power X-band sources is clear and growing forboth science and national security applications On the path to establishing confidence in the reliability of X-band sources for applications requiring 24/7 operation Strong leveraging and working with other Labs Transferring the technology to other labs and industry Program is cost effective and leverages other work to advance the technology base and the uniquehigh power RF competency at SLAC Demonstrating increasing impact on the field Program is critical to SLAC and important for the broad DOE mission SLAC Accelerator Development Program Page 18